In the production of certain semiconductor devices, an epitaxial layer of silicon on a substrate, such as a silicon wafer, is used as the starting material for the devices. The epitaxial layer of silicon is deposited upon the silicon wafer in a chemical vapor deposition (CVD) process wherein the wafer is supported on a silicon carbide-coated graphite susceptor and heated to a high temperature by energy derived from a radio frequency (RF) source. A volatile mixture of silicon is introduced and thermally decomposed or reacted with other gases or vapors at the surface of the wafer to yield silicon which deposits on the wafer surface. Various types of reactors utilizing such RF heated susceptors are well known in the art. One such susceptor utilized in a barrel reactor is described in U.S. Pat. No. 4,099,041 issued to S. Berkman et al. on Jul. 4, 1978 entitled "Susceptor Structure For Heating Semiconductor Substrates".
In recent applications of semiconductor devices, particularly those used for high power circuits, there has been a need to provide a precise and restricted range of resistivity values of the epitaxial layers grown on heavily doped substrates. For example, substrates heavily doped with arsenic having a dopant concentration greater than 5.times.10.sup.19 atoms/cc have been used to provide certain types of power devices, such as rectifiers and FETs, in which the epitaxial layer is required to have a lightly doped concentration of 1.times.10.sup.14 atoms/cc. One problem in the manufacture of such devices is the autogenous contamination that results from the so-called autodoping phenomenon caused by the transfer of the dopant compounds evolved by outdiffusion from the heavily doped substrate into a volatile component form and transferred in such form during the CVD process to the epitaxial layer.
Moreover, it appears that the autodoping effect on individual devices is complicated by cross-contamination doping wherein outdiffused dopants are carried downstream of the input reactant gas flow. The gas is thus contaminated by an increase in its dopant concentration. Eventually the devices downstream of the reactor will receive an epitaxial layer with an increased dopant concentration and thus adversely affect the resistivity of the epitaxial layer.
For the purposes of this description the term "autodoping" is intended to mean the contribution of doping that occurs during the deposition on a doped crystalline substrate of an epitaxial layer derived from dopant products outdiffused from the substrate receiving the epitaxial layer. The term "cross-contamination" is intended to be a form of autodoping in the sense that the cross-contaminating doping products are derived from a different substrate than the substrate receiving the epitaxial layer.
Several attempts have been proposed to reduce the autodoping and cross-contamination effects by, for example, providing on the rear surface of the substrates a sealant coating of oxides of silicon, oxides of silicon nitride and sealant coatings of pure silicon. None of these solutions have been satisfactory or completely successful. Moreover, the extra processing steps of providing such sealant backings to the wafers and, at times, removing them after the epitaxial layer has been deposited, is costly. Attempts to modify the operating parameters of a reactor, such as gas flow, dopant concentration and operating temperatures, have been without success.
There is a need in the art to provide a method for reducing, if not inhibiting, both the autodoping and cross-contamination effects that contaminate epitaxial films during CVD processing.